The invention relates to the plasma display panels, and more particularly to means enabling a so-called xe2x80x9ccell conditioningxe2x80x9d effect to be obtained.
Plasma panels (abbreviated to xe2x80x9cPPxe2x80x9d in the rest of this description) are image display screens of the xe2x80x9cflat screenxe2x80x9d type. There are two main families of PPs: PPs whose operation is of the continuous type, and those whose operation is of the alternative type. All PPs operate using the principle of an electrical discharge in a gas that results in emission of light. They generally include two insulating sheets, each carrying one or more networks of electrodes, the space between the sheets being gas-filled. The sheets are assembled together such that their networks of electrodes are mutually orthogonal. Each intersection of electrodes defines a cell which corresponds to a gas space.
FIG. 1 represents by way of example, in a partial and simplified manner, a classic structure of a color alternative PP. Various types of alternative PPs are found, among which we can mention for example: those of the type using only two crossed electrodes to define and control a cell, as described notably in a French patent published under the number 2,417,848, and those of the so-called xe2x80x9ccoplanar structurexe2x80x9d type whose structure and operation are described for example in the European patent EP-A-0.135.382. Alternative PPs have a common characteristic in that they have an internal memory effect during operation, owing to the fact that their electrodes are covered by layer of dielectric which isolates them from the gas, in other words from the discharge.
In the example of FIG. 1, the PP is of the type having two crossed electrodes defining a cell. It is composed of two substrates or sheets 2, 3, of which one is a front sheet 2, i.e. the sheet that is on the same side as an observer (not shown); this sheet carries a first network of electrodes called xe2x80x9cline electrodesxe2x80x9d, of which only 3 electrodes Y1, Y2, Y3 are shown. The line electrodes Y1 to Y3 are covered with a layer 5 of a dielectric material.
The second sheet 3 forms the rear sheet. It is on the opposite side from the observer and consequently is preferably equipped with components whose function is to prevent the transmission of light towards the observer; it carries a second network of electrodes called xe2x80x9ccolumn electrodesxe2x80x9d, of which only 5 electrodes X1 to X5 are shown in FIG. 1. The two sheets 2, 3, are of a same material, generally glass. These two sheets 2, 3 are destined to be assembled together such that the networks of line and column electrodes are mutually orthogonal.
On the rear sheet 3, the column electrodes X1 to X5 are also covered with a layer 6 of dielectric material. The dielectric layer 6 is itself covered with layers forming bands 7, 8, 9 of luminiferous materials that correspond for example to the colors green, red and blue respectively. The luminiferous bands 7, 8, 9 are placed parallel to and above the column electrodes X1 to X5 from which they are separated by the dielectric layer 6. The rear sheet 3 also includes barriers 11, parallel to the luminiferous bands 7, 8, 9 and placed between them.
The PP is formed by the assembly of the front and rear sheets 2, 3, this operation forming a matrix of cells C1 to Cn. The cells are defined at each intersection between a line electrode Y1 to Y3 and a column electrode X1 to X5. Each cell has a discharge zone whose section corresponds substantially to so-called xe2x80x9cusefulxe2x80x9d areas formed by the surfaces facing the two crossed electrodes. For each cell, the gas discharge causes electric charges that, in the case of an xe2x80x9calternativexe2x80x9d PP, cumulate on the dielectric 5, 6 facing the line and column electrodes; in the example shown here, this is obtained at the rear sheet 3 by means of cavities Ep1 to Epn made in the luminiferous bands 7, 8, 9 substantially opposite the useful areas of the column electrodes X1 to X5.
In the example shown, the intersections made by the first line electrode Y1 with the column electrodes X1 to X5 defines a line of cells, each cell being in the form of a cavity: the first cell C1 is located at the first cavity Ep1, the second cell C2 at the second cavity Ep2, and so on until the fifth cavity Ep5 constituting a fifth cell C5. The first, second and third cavities Ep1, Ep2, Ep3 are respectively located in green 7, red 8 and blue 9 luminiferous bands; they correspond to monochromic cells of three different colors which together can constitute a three-color cell.
The quality of the discharges in each cell, for a given value of the tension applied to the electrodes, depends on the geometry and dimensions of the cells, and the overall quality of the operation of the PP requires that these characteristics must be reproduced with low dispersion for all the cells of the PP. One of these characteristics that is particularly important is the height of the gas space formed between the front and rear sheets 2, 3, when these are assembled.
Generally, in color PPs (which, unlike with monochrome PPs, have luminiferous components enabling them to produce lights of different colors), one of the dimensions of the gas space formed between the sheets 2, 3 corresponds to the distance between these sheets, this distance being defined by the height H1 of the barriers 11; in the rest of this description, these barriers are referred to as xe2x80x9ccarrier barriersxe2x80x9d. During assembly of the two sheets 2, 3, they are separated from each other by the carrier barriers 11 that therefore play the role of spacers.
Since the carrier barriers 11 have the same height H1 as the space separating the sheets 2, 3, they constitute relatively tight partitions, so that in addition to their spacing function mentioned earlier they assure another function known as xe2x80x9cconfinementxe2x80x9d. This well-known confinement function consists notably of confining the discharge, in other words preventing its propagation into non-addressed neighboring cells, and thereby avoiding diaphonic effects between cells, and of preventing the ultraviolet radiation created by a discharge in a given cell from exciting the luminiferous material in adjacent cells, which would lead to lack of saturation of the colors, this phenomenon being known as a diaphoty effect. We note in the example shown in FIG. 1, that since the carrier barriers 11 are placed so as separate two luminiferous bands 7, 8, 9 of different colors in a so-called xe2x80x9ctriadxe2x80x9d arrangement, they assure these confinement functions only between cells situated along the same line electrode Y1 to Y3, as for cells C1 to C5.
However the authors of the invention have observed that excessive confinement of cells can in some cases adversely affect the operation of the PP, especially when high speed triggering or inscription of the cells is necessary, as for example in the case of television images. The authors realize that a structure such as the one shown in FIG. 1, leads to total confinement of the cells, in other words to total isolation between two adjacent cells of different colors, and that this total confinement may deprive these cells of the benefits of phenomena of transfer, from one cell to another, of charges (ions or electrons of the plasma) and/or ultraviolet photons that can contribute to the triggering of a discharge in the gas.
Such transfer phenomena produce an effect known as xe2x80x9ccell conditioningxe2x80x9d, which can occur only if the structure of the cells leaves a path in the gas-filled space between neighboring cells, and in both directions, in other words along the line electrodes and along the column electrodes. Total confinement of a cell with respect to its two neighboring cells of different colors prevents this conditioning effect and reduces the cell activation speed.
The present invention proposes simple means of assuring in a PP the confinement and cell conditioning functions mentioned above, but without adversely affecting the constancy of the spacing between the two sheets of the PP. The invention achieves this notably by separating the sheet spacing function from the cell confinement function, in order to be able to modulate the action of the means of confinement.
The invention is therefore a plasma display panel comprising two parallel sheets assembled one on top of the other, at least two networks of electrodes defining cells, means of spacing defining a spacing distance between the sheets, and means of confinement of the cells, characterized in that the means of confinement are barriers whose height is perpendicular to the sheets, said height being less than the spacing distance between the sheets.